1. Field of the Invention
The present invention relates generally to an external electrode fluorescent lamp, a liquid crystal display backlight unit using the same, and a device for driving the external electrode fluorescent lamp, and more particularly to a liquid crystal display backlight unit using an external electrode fluorescent lamp, which can easily produce a surface light source with higher brightness and brightness uniformity than a conventional edge-type or a direct-type backlight unit, reduce a calorific value of a liquid crystal display panel due to electrodes of a fluorescent lamp, prevent breakdown of a fluorescent lamp caused by breakdown of electrodes, and extend the life of a fluorescent lamp, and further particularly, to an external electrode fluorescent lamp, liquid crystal display backlight unit using the same and device for driving an external electrode surface emission fluorescent lamp, which can simplify the manufacturing process and improve the productivity thereof, and which can be easily applied to a large-scale backlight unit.
2. Description of Related Art
Generally, a liquid crystal display (LCD) used as display means for characters, graphics and moving pictures has been greatly highlighted as a next generation display device for mobile phones or televisions because it causes less fatigue of eyes than a conventional cathode ray tube (CRT) display device, and it can realize miniaturization, light weight, and low power consumption.
The construction of a conventional LCD panel in which characters or images are displayed on LCD is described in brief. First, if liquid crystal material is injected between a pair of surface-processed transparent glass plates, and an electrical signal (voltage) is supplied to the injected liquid crystal material using an LCD driving circuit (not shown) for generating a driving signal, phase variation of the liquid crystal material occurs by the electrical signal. The LCD driving circuit applies different voltages to the liquid crystal material to vary distribution of the liquid crystal material, thus enabling specific characters or images to be displayed.
However, since an LCD panel on which characters are displayed cannot emit light for itself, a means for assisting in visually recognizing contents (characters or logos) displayed on the panel is required. Currently, a backlight system using lamps which irradiate light from the sides or the back of a LCD panel is generally used as the assisting means.
Conventional backlight systems are mainly classified into edge-type backlight units and direct-type backlight units according to positions of fluorescent lamps projecting light. The edge-type backlight units employ a manner in which light sources are positioned beneath both sides of the panel, such that light inputted from the light sources forms a surface light source by a light guide plate and a reflective sheet and the surface light source illuminates cells of the LCD panel. Such an edge-type backlight unit is advantageous in that, since it indirectly guides light radiated from the light sources, brightness uniformity is high. However, it is problematic in that brightness decreases relative to the brightness uniformity.
FIG. 1 shows an embodiment of a conventional edge-type backlight unit. Referring to FIG. 1, a lamp cover for covering fluorescent lamps, the fluorescent lamps for radiating light by the supply of power, a reflective sheet for reflecting the radiated light in a predetermined direction, a light guide plate for guiding the radiated light, a diffusion sheet for uniformly radiating incident light to prisms, a vertical prism, a horizontal prism, and a protective sheet are layered in order from the bottom. In the above-described edge-type backlight unit, since the fluorescent lamps are positioned at the side of the light guide plate, brightness uniformity increases, while brightness decreases.
Further, the direct-type backlight units employ a manner in which light sources (cold cathode fluorescent lamps) are arranged beneath an LCD panel, a diffusion sheet is arranged on the front of the light sources, and a reflective sheet is arranged on the back of the light sources, such that light radiated from the light sources is reflected and diffused to be irradiated onto cells of the LCD panel. Since such a direct-type backlight unit effectively uses light using the reflective sheet and the diffusion sheet, it can obtain high brightness, so it is suitable for backlight units requiring high brightness. However, the direct-type backlight unit is problematic in that it cannot provide sufficient brightness according to the size of LCD panels which become large, and brightness uniformity is also decreased.
Moreover, the conventional direct-type backlight unit requires as many inverters as the number of fluorescent lamps used as light sources. That is, characteristics of respective fluorescent lamps used as the light sources are slightly different. Therefore, in the case where the fluorescent lamps are connected in parallel with each other, there occurs a problem that a plurality of fluorescent lamps are not simultaneously turned on due to the difference in discharge properties, if one inverter having a high power supplying capability is mainly used. That is, some of fluorescent lamps may be turned on, and the remaining fluorescent lamps may be turned on late or turned off. In order to solve the problem, inverters are respectively connected to fluorescent lamps to drive the fluorescent lamps. However, there are problems, such as high power consumption, cost increase due to the increased number of inverters, and productivity decrease due to the increased assembly time, degradation of LCD due to heat generated by electrodes, etc.
Further, a prior art, plate-type surface emission fluorescent lamp applied by the present applicant, improves brightness uniformity and brightness of conventional light sources (fluorescent lamps) for backlighting. FIG. 12 is a plan view of a previously applied surface emission fluorescent lamp. As shown in FIG. 12, an upper sheet of the lamp is constructed such that serpentine-shaped channels into which discharge gas is injected and which are isolated from the outside are arranged to be adjacent to each other. Further, bent portions are mutually connected, such that a single channel is formed in the upper sheet. At both ends of the single channel, internal electrodes 201 are installed.
FIG. 13 is a sectional view by A-A line of the surface emission fluorescent lamp 203 of FIG. 12, and shows sections of channels 203a formed adjacent to each other. Actually, the channels 203a shown to be separated respectively are mutually connected to each other to form a single path. In FIG. 13, although the section “A” is depicted by a semicircle, the shape of the channels 203 can be varied to a rectangle, a diamond, etc.
Further, FIG. 14 is a sectional view by B-B line of FIG. 12, and shows the installation of the internal electrodes 201 of the surface emission fluorescent lamp 203. As shown in FIG. 14, one end of each of the internal electrodes 201 is inserted into the surface emission fluorescent lamp 203. Therefore, many manufacturing processes are added to insert and fix the internal electrodes 201.
As described above, the previously applied surface emission fluorescent lamp 203 uniformly radiates light over the entire surface area, thus supplementing the disadvantages of the conventional edge-type and direct-type backlight units to provide high brightness and high brightness uniformity. Especially, since the surface emission fluorescent lamp 203 has serpentine-shaped channels, the brightness and the brightness uniformity are remarkably improved. Further, the shape of the surface emission fluorescent lamp 203 can be changed to “L”, “W”, etc. In this case, the upper sheet of the lamp is typically formed in the shape of “L” or “W”, and the lower sheet thereof is formed in the shape of a plate, such that the upper and lower sheets are manufactured to be combined with each other, or to be integrated.
However, as the construction of the fluorescent lamp is varied as described above, there are inconveniences in that installation positions of the internal electrodes 201 for supplying power to the fluorescent lamp are frequently varied, so manufacturing equipment must be changed. Moreover, the internal electrodes 201 are fixedly inserted into the surface emission fluorescent lamp 203, thus causing several problems, such as increase of the manufacturing process of the fluorescent lamp and the deterioration of productivity due to breakdown of internal electrodes, etc. Further, the surface emission fluorescent lamp is problematic in that, if a plurality of fluorescent lamps are connected in parallel to apply the fluorescent lamps to a large-scale backlight unit, wiring is complicated to connect inverters to respective electrodes, so the volume of the backlight unit increases.